1. Field of the Invention
This invention is directed to methods and systems for purifying polymers and mixing polymers with active agents used for medical devices, such as drug eluting and/or delivery stents.
2. Description of the State of the Art
Polymeric materials have a variety of applications in the medical field, including with stents. Metallic stents are being coated with a polymeric material for local delivery of a drug or therapeutic substance. Stents are also being made from polymeric materials that are bioabsorbable or bioerodable so as to maintain vascular patency for a desired duration of time until they are absorbed or degraded and eliminated by the body. Biodegradable stents can also contain or carry therapeutic substances. Polymers can contain impurities that trigger adverse biological responses when implanted into the body. Polymers can contain impurities such as catalysts, initiators, processing aids, suspension aids, unreacted monomers and oligomers or other low molecular weight species, even though the polymer may be classified as a “food packaging grade” polymer by the manufacturer. Accordingly, there is a need for purifying polymers for medical applications.
In addition to containing impurities, another impediment faced with drug eluting and/or delivery stents is the inconsistency of replicating stents having the same drug profile or producing a stent having a homogeneous or substantially homogenous drug concentration throughout the stent body or a segment of the body of the stent. “Homogeneous” may be defined as uniform in chemical composition, appearance and properties. Current processes can lead to non-homogeneous polymer-drug compositions that form higher concentration of drugs on certain parts of a stent than other areas of the stent. For example, a coating for a drug-eluting and/or delivery stent is typically prepared by dissolving a polymer and a drug in a suitable solvent or combination of solvents. The polymer and active agent may be dissolved together or separately in their respective solvents then added together. The latter is a more effective technique if the polymer and the drug cannot be easily blended. Operations such as stirring, heating and/or agitation are employed to combine the polymer with the active agent. The foregoing method of blending the polymer and the active agent does not produce a desired degree of homogeneity. The difference in density between the active agent and the polymer may make adequate mixing of the ingredients difficult to achieve. Inadequate mixing may result in a coating with different active agent concentration in various portions of the body of the device. Consistency is replication is also compromised.
Furthermore, heating of the composition may be needed to effect dissolution. Some drugs degrade or diminish in property when exposed to elevated temperatures. The degradation of drugs also depends on the time of exposure. Therefore, it is important that an agitation method limit exposure of a polymer and the active agent solution to high temperatures and the time the solution is exposed to such temperatures. The embodiments of the present invention address these as well as other needs.